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I know of no way to guarantee that a piece is returned with a phone number on it. How is that the responsibility of a Direct Mail Lead provider, and what kind of credit would you expect?
EDIT:
Would a system, that found the individual, that you could access online, that did a phone lookup for you on the fly, be a reasonable compromise?
I would expect to not pay for any lead that did not have a valid contact number. You can't know in advance but once it's reported to you there should be a lead credit.